Clinching the deal

FAST was recently speaking to BTM (UK) Automation Products managing director Mark Firmin about the company’s clinching process, the renowned TOG-L-LOC sheet metal joining system and its offspring – along with the future

What exactly is clinching?

Clinching is a high-speed mechanical cold forming technique for joining sheet metal production components without the need for additional fasteners, using only the parent metal. The clinching process uses inexpensive punch and die tooling to create leak-proof joints in a single press stroke.

Compared to other joining techniques, clinching is a fast process, easy to automate and so particularly suited when high productivity is needed; moreover, since no welding material is used, there are no supply or storage costs and chemical contamination of the joint is lower. The absence of fumes, emissions or high currents also ensures high safety for the operator and a low environmental impact. With standard Tog-L-Loc (circular) tooling, we are able to join two sheets up to a maximum metal thickness of 8.0mm; with the Lance-N-Loc (rectangular) tooling multiple layers, again up to 8.0mm is achievable.

What material types is clinching most suited to?

Normally used for joining ductile materials, such as mild steel and aluminium, but can also join low grade stainless steels, brass & copper. The metal combinations do not need to be identical as dissimilar metals can be joined, allowing a vast range of products to be successfully joined.

How is clinching applied?

Depending on production volumes, clinching can be applied via many methods. Portable hand-held units for working around larger parts, single or twin-joint presses are commonly used for lower volumes, with multi-joint Die Sets and special designed /built machines for high volume high-speed production. Robot applications, especially in the automotive industry, are quite standard nowadays. We can even supply adapters, so that our tooling can be used on PEM & Haeger presses, which offers the end user a broader range of uses for this equipment.

Is it an alternative to riveting or bolting or bonding?

Yes, it is – and, if I compare it to each of the above, then first for riveting and with bolting, the clinch process only uses the material that is there. No additional fasteners, such as the rivet or bolt, are used, resulting in reduced costs, but the clinch joint is only as strong as the metal itself. With bonding, clinching is vastly less expensive. But, if you require a super high-strength joint, then use a combination of clinching and bonding.

If you had to summarise the top three reasons for design engineers to consider the advantages of clinching, what would they be?

  • Weight factor – clinching uses no third part fasteners, so there is no additional weight to the component
  • Speed of application – takes less than 1 second to complete
  • Highly fatigue resistant – trials have shown that a clinch joint over an accelerated lifecycle test has negligible loss in strength, compared to a spot-weld.

What other advantages should potential users be aware of?

  • More economical – much less energy consumption compared to a spot-weld joint for example
  • Corrosion resistant – salt tests have shown a very high resistance
  • No thermal considerations during the joining process resulting in zero distortion
  • Very high joint strength when used in conjunction with a structural adhesive
  • Long tool life – in excess of 300,000 joints is common
  • Non-destructive testing making quality control very easy
  • Extremely high joint repeatability
  • Versatile – dissimilar metals can be joined.

Can you tell us anything about typical applications where the process has been used?

In the HVAC industry, we have designed and built a portable clinch unit for joining the flange onto the end of the duct. This is typically three layers of galvanised mild steel up to 3.6mm total thickness, which we join with the rectangular Lance-N-Loc joint. This joint has been successfully issued with a TÜV Nord certificate for air tightness.

For office partitioning, we have designed and built a special-purpose machine for joining several length options of door lintels. Each lintel is made from anodised aluminium (1.5mm thick) and is joined to a steel bracket at each end (1.0mm thick), with two joints per bracket. This machine consists of two hydraulic cylinders, one static and one that is manually moved to a predetermined position and, upon pressing the start button, four clinch joints are simultaneously completed. Prior to the purchase of this machine, the customer was drilling holes and screwing the bracket to the lintel.

Is it possible to clinch pre-coated materials?

It is – and very common. Zinc, galvanised and pre-painted metals can all be joined with the coating only being slightly thinned as the metal is being drawn down into the clinch die. The result is that it is not necessary to redress or repaint the component after the clinch process is complete, unlike some other joining methods.

If a customer specifies clinching for Project A, can the equipment be subsequently adapted to be used on other projects that may follow?

In most cases, yes. If it is a case of the metals being either thicker or thinner, then sometimes the same tooling can be used (if the thickness has not changed dramatically), but the worst case scenario is normally the clinch die is changed. Clearly, if the component has changed – ie, larger flanges are now used and we need to overcome this – then the equipment will need to be changed. But normally clinching is a very forgiving method of joining metal.

Where do you see clinching going and what are the clinching hurdles still to be overcome?

The joining of harder and more exotic materials is the next step as these are being used more and more, especially in the automotive and aerospace industries where the need to save weight is great. BTM have already been successful in joining spring steel components. It is not just a case of being able to join the metal; it is also a case of finding the right metals to construct the clinch tools to be able to clinch these new materials, which is the challenge we face.

Have you any new products either just about to be released or have just been?

Yes, we do. In 99% of most applications, the clinch joint is never seen; but, in some cases, the customer requires a more `pleasing to the eye` type of joint, so BTM has developed what is called the V-Loc (short for visual). V-Loc is a visually improved Tog-L-Loc joint that features a raised spherical inner diameter with a concentric outer ring. This design is intended to give the appearance of a more traditional fastener, but the process is essentially the same as the trusted Tog-L-Loc joining method.

Also, we now have the Oval-Loc, a sort of rounded-off Lance-N-Loc joint, which was designed primarily for situations where only a single joint is required, but does not allow the two sheets of metal to rotate.